package/qemu: do not use autotools for qemu

Although QEMU has a ./configure script, it is not an autotools-package,
so switch to providing build and install commands, and to using
generic-package.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>